As power and performance of semiconductor devices increase, so does the need for effective heat dissipation. Various methods of providing heat dissipation in semiconductor devices are known. One such method is to attach a heat sink external to a package body. Another known method is to incorporate a heat sink internal to a molded package body. Yet another practiced method of removing heat is to draw heat from leads of a device to a cooled substrate. Also, forcing cooled air or liquid over a device has been used for heat dissipation. These and other widely accepted methods for removing heat generally have at least two common objectives. One objective is to remove heat from as large a surface area as possible. If using a heat sink, this often involves forming contours, grooves, fins, or other features into the heat sink which increase surface area. The second objective is to position the heat removal medium, whether it be a heat sink, air, or a cooled liquid, as close to the source of heat as possible, or in other words to establish a path of least thermal resistance. In semiconductor devices, a semiconductor die and more particularly an active surface of the die is the largest source of heat generation.
A problem with many of the existing methods for removing heat in semiconductor devices is that the methods cannot be used effectively in thin or low-profile packages. An example of a low-profile package in which traditional heat removal methods are not effective is an over-molded pad array carrier (OMPAC) device. Existing OMPAC devices typically include a thin printed circuit board (PCB) substrate on which a semiconductor die is mounted. The die is electrically coupled to conductive traces formed on a top surface of the PCB. Each conductive trace is routed to a corresponding conductive trace on the bottom surface of the PCB by a conductive via which extends through the PCB. The traces on the bottom surface of the PCB each terminate at a conductive pad to form an array of pads on the bottom of the PCB (thus the term "pad array" device). The semiconductor die and the top of the PCB are encapsulated by a thin molding compound to form a package body. A solder ball is attached to each pad on the bottom of the PCB to provide external electrical accessibility to the encapsulated die.
In achieving the two objectives discussed above using traditional heat removal methods, many desirable attributes of OMPAC devices are compromised. For instance, adding a heat sink to the top of a package body to achieve a large exposed heat sink surface area in an OMPAC device undesirably increases the height or thickness of the device. To minimize a height increase, a heat sink may be positioned within a recess in the package body; however, this increases the chance of electrically short-circuiting the heat sink to wire bonds in the device. As another example, mounting a semiconductor die directly to a heat sink using conventional methods prohibits routing conductive traces underneath the die, an attractive feature which enables OMPAC devices to occupy minimal board area on a user's board.
One proposed method of avoiding the problems presented above in removing heat from OMPAC devices is to use "dummy" conductive vias to remove heat. The "dummy" conductive vias, also known as thermal vias, are typically plated vias which are not used to transmit electrical signals but which instead transmit heat away from an OMPAC device to a surrounding ambient. In most instances, thermal vias are positioned directly beneath a semiconductor die. Using such thermal vias has several disadvantages, the primary disadvantage being the limited area for removing heat. Typically, each plated via has a cross-sectional area of conductive material on the order of only 0.02 mm.sup.2. Therefore, in order to effectively remove heat in a semiconductor device, a large number of thermal vias is required. Since thermal vias occupy space which would otherwise be used as signal vias, semiconductor manufacturers are reluctant to use a large number of thermal vias so that overall device size is not unnecessarily increased. Another disadvantage in using thermal vias is the potential for reliability problems. Since the conductive vias are hollow, a path for moisture and contamination exists directly beneath the die. Typically, the only impediment to contamination through thermal vias is a thin solder resist mask. However, solder resist mask materials are known to absorb moisture.